int main(){
    cv::Mat img=cv::imread("/home/nan/图片/highdeepth/starry.jpg",cv::IMREAD_REDUCED_COLOR_8);
     // imread( const String& filename, int flags = IMREAD_COLOR );
    if(img.empty()) return -1;
    cv::imshow("BGR",img);
    std::vector<cv::Mat> planes;
    cv::split(img,planes);
    cv::imshow("B",planes[0]);
    cv::imshow("G",planes[1]);
    cv::imshow("R",planes[2]);
    cv::imshow("BGR",img);

    img=cv::imread("/home/nan/图片/highdeepth/starry.jpg",cv::IMREAD_REDUCED_GRAYSCALE_8);
    cv::split(img,planes);  // 灰度图只能分离出一个通道
    cv::imshow("Gray",planes[0]);
    cv::waitKey(0);
    return 0;
}

注意：如果图像像素过大，并且要读出全部图像的话可以使用尺寸缩减 如：cv::IMREAD_REDUCED_COLOR_8。

flags包括以下内容：

　　　　IMREAD_UNCHANGED            = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).
       IMREAD_GRAYSCALE            = 0,  //!< If set, always convert image to the single channel grayscale image.
       IMREAD_COLOR                = 1,  //!< If set, always convert image to the 3 channel BGR color image.
       IMREAD_ANYDEPTH             = 2,  //!< If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
       IMREAD_ANYCOLOR             = 4,  //!< If set, the image is read in any possible color format.
       IMREAD_LOAD_GDAL            = 8,  //!< If set, use the gdal driver for loading the image.
       IMREAD_REDUCED_GRAYSCALE_2  = 16, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/2.
       IMREAD_REDUCED_COLOR_2      = 17, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.
       IMREAD_REDUCED_GRAYSCALE_4  = 32, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/4.
       IMREAD_REDUCED_COLOR_4      = 33, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
       IMREAD_REDUCED_GRAYSCALE_8  = 64, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/8.
       IMREAD_REDUCED_COLOR_8      = 65, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.
       IMREAD_IGNORE_ORIENTATION   = 128 //!< If set, do not rotate the image according to EXIF's orientation flag.